1. Field of the Invention
The present invention relates to an electrical machine, and, more particularly, to a three-phase generator including a stator, a rotor with an excitation coil, a rotary transformer with primary and secondary windings, which are arranged on respective cores of soft ferrite material and are magnetically coupled with each other by an air gap, and a control circuit for loading the primary winding of the rotary transformer with an alternating current.
2. Description of the Related Art
The basic structure of this type of electrical machine is known from a number of prior art references, for example from German Patent Application DE 199 35 074 A1. The three-phase generator described there has a stationary stator, a rotatable rotor with an excitation coil and a transformer with a primary and a secondary winding. The primary and secondary windings are each arranged on a soft ferrite core and are coupled with each other via an air gap. The rotary transformer delivers excitation power to the excitation coil. The primary winding of the rotary transformer is loaded with an alternating current with the help of a control circuit, which is converted and rectified on the secondary side of the rotary transformer with a rectifier circuit, for example a two way rectifier or a rectifier bridge circuit.
A common problem in the design and operation of this sort of rotary transformer is the adherence to a certain maximum temperature, in order to avoid overheating of the rotary transformer. A temperature sensor is built into the rotary transformer for this purpose in the state of the art. The temperature of the rotary transformer is detected with the help of the temperature sensor. When the temperature of the rotary transformer climbs to a prohibited value, the transformer power is reduced by a suitable control means.
This type of active control fulfills its purpose, however special components, such as the temperature sensor, suitable signal processing circuits and a controller for changing the transformer power, for example by an adjustment of an on/off ratio or key ratio, are necessary.
It is an object of the present invention to provide an electrical machine of the above-described type, especially a three-phase generator, which is improved so that the temperature is limited to avoid impermissibly high or excess temperatures without including a temperature sensor in the apparatus.
This object and others which will be made more apparent hereinafter are attained in an electrical machine, especially a three-phase generator, which comprises a stator, a rotor with an excitation coil, a rotary transformer with primary and secondary windings, which are arranged on respective cores of soft ferrite material and are magnetically coupled with each other via an air gap and a control circuit for loading the primary winding of the rotary transformer with an alternating current.
According to the invention the control circuit includes an input-side current limiting circuit, whereby an integrated temperature control is obtained in cooperation with the Curie effect occurring in the soft ferrite material of the respective cores of the primary and secondary windings.
The control circuit of the invention with the input-side current limiting circuit makes use of the fact that soft ferrite material in the vicinity of its Curie temperature looses its magnetic properties. Accordingly the magnetic permeability drops to a value equivalent to 1 at the Curie temperature. The ferrite material behaves like air and energy can no longer be transferred from the primary winding of the transformer to the secondary winding.
The principal advantages of the invention include a reliable control of excessive temperatures for the ferrite material and a reliable temperature regulation without the need of a temperature sensor.